Amorphous alloys have generally been prepared by rapid quenching from above the melt temperatures to ambient temperatures. Generally, cooling rates of 105° C./sec have been employed to achieve an amorphous structure. However, at such high cooling rates, the heat can not be extracted from thick sections, and, as such, the thickness of articles made from amorphous alloys has been limited to tens of micrometers in at least in one dimension. This limiting dimension is generally referred to as the critical casting thickness, and can be related by heat-flow calculations to the cooling rate (or critical cooling rate) required to form an amorphous phase.
This critical thickness (or critical cooling rate) can also be used as a measure of the processability of an amorphous alloy. Until the early nineties, the processability of amorphous alloys was quite limited, and amorphous alloys were readily available only in powder form or in very thin foils or strips with critical dimensions of less than 100 micrometers. However, in the early nineties, a new class of amorphous alloys was developed that was based mostly on Zr and Ti alloy systems. It was observed that these families of alloys have much lower critical cooling rates of less than 103° C./sec, and in some cases as low as 10° C./sec. Accordingly, it was possible to form articles having much larger critical casting thicknesses of from about 1.0 mm to as large as about 20 mm. As such, these alloys are readily cast and shaped into three-dimensional objects, and are generally referred to as bulk-solidifying amorphous alloys (“B-SA Alloys”). Recently, several new classes of B-SA Alloy have been discovered which include Pt-base, Fe-base etc.
The unique properties of B-SA Alloys includes very high strength, high specific strength, large elastic strain limit, and high corrosion resistance that make them interesting for structural applications. However, B-SA Alloys show relatively limited ductility and low toughness compared to their high yield strength values. For example, when a strip of B-SA Alloy having a 2.0 mm thickness is subjected to loading at room temperature, very little (less than 2% if any) plastic deformation takes place upon yielding before failure. Upon yielding, B-SA Alloys tend to form shear bands in which plastic deformation occurs in a highly localized manner. In an unconfined geometry, failure of the B-SA Alloys typically occurs along a single shear band that cuts across the sample at an angle of 45° (the direction of maximum resolved shear stress) with respect to the compression axis. This limits the global plasticity of B-SA Alloys in unconfined geometries to less than 1%, and restricts the use of B-SA Alloys as structural materials for most applications. Furthermore, B-SA Alloys show relatively lower resistance to crack propagation, which precludes the effective use of their high yield strength values.
Additional challenges are encountered in using B-SA Alloys for precious metal applications. For example, although the overall properties of B-SA Alloys makes Pt-base B-SA Alloys attractive for jewelry applications, jewelry accessories made from amorphous platinum alloy have to withstand temperatures up to 200° C. In order to use the alloy for jewelry accessories it has to maintain its amorphous nature up to 200° C. This means that the glass transition temperature should be above 200° C. On the other hand, the glass transition temperature should be low in order to both lower the processing temperature and minimize shrinkage due to thermal expansion. In addition, Pt-rich bulk amorphous alloys have compositions dose to the eutectic compositions. Therefore, the liquidus temperature of the alloy is generally lower than the average liquidus temperature of the constituents. Bulk solidifying amorphous alloys with a liquidus temperature below 1000° C. or more preferably below 700° C. would be desirable due to the ease of fabrication. Reaction with the mold material, oxidation, and embrittlement would be highly reduced compare to the commercial crystalline Pt-alloys.
Trying to achieve these properties is a challenge in casting commercially used platinum alloys due to their high melting temperatures. For example, conventional Pt-alloys have melting temperatures generally above 1700° C. This high melting temperature causes serious problems in processing. At processing temperatures above the melting temperature the Pt alloy react with most investment materials which leads to contamination, oxidation, and embrittlement of the alloy. To process alloys at these elevated temperatures sophisticated expensive equipment is mandatory. In addition, during cooling to room temperature these materials shrink due to crystallization and thermal expansion. This leads to low quality casting results. In order to increase the properties subsequent processing steps such as annealing are necessary. Another challenge in processing commercial crystalline Pt-alloys is that during crystallization the alloy changes its composition. This results in a non-uniform composition in at least at portion of the alloy.
Accordingly, a need exists to develop highly processable bulk solidifying amorphous alloys with high ductility, such as platinum rich compositions for jewelry applications. Although a number of different bulk-solidifying amorphous alloy formulations have been previously disclosed, none of these formulations have been reported to have the desired processability and improved mechanical properties, such as those desired in jewelry applications.